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The American Society of Automotive Engineers defines two categories of engine oil viscosity, one which deals with high temperatures typically reached in a modern engine, and one with winter start conditions. SAE numbers 20 to 60 cover five viscosity ranges based on measurements at 100°C. Various types of laboratory kit are used to measure viscosity, but the answer comes out in standard units known as "Centistokes"(cSt).

SAE 20 covers 5.6 to below 9.3 cSt at 100°C
SAE 30 covers 9.3 to below 12.5 cSt
SAE 40 covers 12.5 to below 16.3 cSt
SAE 50 covers 16.3 to below 21.9 cSt
SAE 60 covers 21.9 to below 26.1 cSt

"W" ratings are only relevant to sub-zero cold start conditions.
"W" stands for Winter. (Not "weight" as saloon bar experts will tell you).

A 0W has to be below 3250 cSt viscosity @ -30°C
A 5W has to be below 3500 cSt viscosity @ -25°C
A 10W has to be below 3500 cSt viscosity @ -20°C
A 15W has to be below 3500 cSt viscosity @ -15°C
A 20W has to be below 4500 cSt viscosity @ -10°C